Data unit detection including antenna diversity

ABSTRACT

A system adjusts a receiving device in response to sensing symbols. An automatic gain control is adjusted in response receiving a first symbol of a data unit from a first antenna. After adjusting the automatic gain control at least a second and a third symbol of the data unit are received, and in response thereto (1) a first energy of at least one of the second and third symbols is calculated, (2) a first frequency offset of at least one of the second and third symbols is calculated, and (3) a first temporal offset of at least one of the second and third symbols is calculated. The automatic gain control in response receiving a fourth symbol of the data unit from a second antenna is received. After adjusting the automatic gain control at least a fifth and a sixth symbol of the data unit is calculated, and in response thereto (1) a second energy of at least one of the fifth and sixth symbols is calculated, (2) a second frequency offset of at least one of the fifth and sixth symbols is calculated, and (3) a second temporal offset of at least one of the fifth and sixth symbols is calculated. At least one of the first and second antenna is selected based upon a comparison between the first and second energy. In this manner, antenna diversity selection may be performed within a limited duration of available symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/141,419, filed Jun. 29, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to a communication system includingantenna selection diversity.

The Institute of Electrical and Electronic Engineers (IEEE), DRAFTSUPPLEMENT TO STANDARD FOR INFORMATION TECHNOLOGY-TELECOMMUNICATIONS ANDINFORMATION EXCHANGE BETWEEN SYSTEMS—LOCAL AND METROPOLITAN AREANETWORKS—SPECIFIC REQUIREMENTS—PART 11: WIRELESS LAN MEDIUM ACCESSCONTROL (MAC) AND PHYSICAL LAYER (PHY) SPECIFICATIONS: HIGH SPEEDPHYSICAL LAYER IN THE 5 GHz BAND, IEEE P802.11A/D7.0, July 1999, is partof a family of standards for wireless Local and Metropolitan AreaNetworks (hereinafter LAN). The proposed standard specifies certaincharacteristics of a high speed, digital, wireless communication LANbased on Orthogonal Frequency Division Multiplexing (OFDM) and packetswitching, incorporated by reference herein.

In an IEEE 802.11A network, data is transferred in data units thatinclude a header and a data section. The data unit may be any generaldata structure, sometimes referred to as a packet or frame. The headerof each data unit includes a preamble or OFDM training structurecomprising a “short training sequence” followed by a “long trainingsequence.” The “long training sequence” comprises two 3.2 μs durationsymbols. It is to be understood that symbols may be any type of signal,different durations, different amplitudes, different frequencies, anddifferent characteristics, as desired. The long training sequence isused for channel and fine frequency offset estimation. The shorttraining sequence comprises ten repetitions of a 0.8 μs duration symbolfor a total sequence length of 8 μs. During the short training sequencethe receiver normally performs signal detection, automatic gain control(AGC), coarse frequency offset determination (CFOD), and timingsynchronization. In addition, the receiving device may perform energydetermination and antenna diversity selection.

A detection circuit of a receiving device converts an analog radiofrequency (R/F) signal received at the antenna to a digital signal anddetermines whether the received signal is sufficiently strong to berecognizable above the noise in the communication system. The signaldetection circuit senses the presence of a signal. The strength of thereceived R/F signal can vary by orders of magnitude. On the other hand,the analog-to-digital (A/D) signal converter of the detector requires arelatively constant amplitude input signal to avoid clipping and loss ofmessage bits. Typically, an automatic gain control (AGC) circuitcontrols the amplitude variation of the R/F signal at the input to theA/D converter while the amplitude of the received R/F signal is varying.

The transmitting device and the receiving device each include a clockcircuit, normally implemented as an oscillator. In order to synchronizethe frequency relationship of the transmitted and received signals, theCFOD circuit synchronizes the frequency of the oscillator in thereceiving device to match that of the received signals. In this mannerthe receiving device adjusts the oscillator to match the actualfrequencies of the received symbols. In order to synchronize thetemporal relationship of the transmitted and received signals, thetiming synchronization circuit synchronizes the temporal relationship ofthe oscillator in the receiver to match that of the received signals. Inthis manner the receiving device determines where each symbol actuallystarts.

With high transmission frequencies, such as in the range of 5-6 GHz, theresulting wavelength of the signal is on the order of five centimeters.With such a short wavelength the receiving device, such as a wirelesstelephone, may be periodically located in an unsuitable phaserelationship with respect to the received signal. In other words, thereceiving device may be positioned at a location where the signal is ata minimum making reception difficult, if at all possible. Accordingly,it is preferable to include multiple antennas interconnected to the samereceiving device at spaced apart locations. With multiple spaced apartantennas it is likely that at least one antenna will sense a strongsignal. Normally the antenna sensing the strongest signal is selected toreceive the following data unit.

Referring to FIG. 1, one possible receiving device 20 includes a pair ofspaced apart antennas 22 a and 22 b. Each of the antennas 22 a and 22 bis interconnected to a respective detection circuit 24 a and 24 b. Eachof the detection circuits 24 a and 24 b performs signal detection,automatic gain control (AGC), energy determination, coarse frequencyoffset determination (CFOD), and timing synchronization. The energydetermination determines which antenna 22 a, 22 b senses the strongestsignal, normally using a correlator, and accordingly a switch circuit 26selects the antenna 22 a, 22 b with the strongest signal to receive thefollowing data unit. Unfortunately, including a pair of detectioncircuits 24 a, 24 b within the receiving device 20 consumes twice thepower of a single detection circuit and increases the expense of thereceiving device 20.

What is desired, therefore, is a receiving device that includes antennadiversity with a single detection circuit, especially a receiving devicesuitable for a P802.11A.

BRIEF SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks of theprior art by providing a system for adjusting a receiving device inresponse to sensing symbols. An automatic gain control is adjusted inresponse receiving a first symbol of a data unit from a first antenna.After adjusting the automatic gain control at least a second and a thirdsymbol of the data unit are received, and in response thereto (1) afirst energy of at least one of the second and third symbols iscalculated, (2) a first frequency offset of at least one of the secondand third symbols is calculated, and (3) a first temporal offset of atleast one of the second and third symbols is calculated. The automaticgain control in response receiving a fourth symbol of the data unit froma second antenna is calculated. After adjusting the automatic gaincontrol at least a fifth and a sixth symbol of the data unit isreceived, and in response thereto (1) a second energy of at least one ofthe fifth and sixth symbols is calculated, (2) a second frequency offsetof at least one of the fifth and sixth symbols is calculated, and (3) asecond temporal offset of at least one of the fifth and sixth symbols iscalculated. At least one of the first and second antenna is selectedbased upon a comparison between the first and second energy. In thismanner, antenna diversity selection may be performed within a limitedduration of available symbols.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic illustration of a receiving device including twodetection circuits.

FIG. 2 is a schematic illustration of a receiving device including onedetecting circuit.

FIG. 3 is a simplified block diagram of a detecting circuit.

FIG. 4 is a timing diagram for receiving a training sequence.

FIG. 5 is an exemplary timing diagram of receiving a training sequencein accordance with the present invention.

FIG. 6 is an exemplary flowchart for the training sequence of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2, a receiving device 30 includes one detectingcircuit 32 switched between two or more spaced apart antennas 34 a, 34b. In this manner the receiving device 30 only requires a singledetecting circuit 32 thereby reducing power consumption and expense. TheP802.11A standard requires the determination of many parameters with theten short symbols provided during the short training sequence. Referringalso to FIGS. 3 and 4, the first symbol is sensed by a detector circuit36 of the detecting circuit 32 from one of the antennas 32 a, 32 b tosense a data unit. The second symbol sensed by the detector circuit 36is used by the AGC circuit 38, as previously described. The AGC circuit38 may require one or more symbols. The third and fourth symbols sensedby the detector circuit 36 are used by the energy determination circuit40 to measure the signal strength. Next, a switch circuit 42 switches tothe other antenna 32 a, 32 b during the fifth symbol. The sixth symbolis then sensed by the detector circuit 36. The seventh symbol sensed bythe detector circuit 36 is used by the AGC circuit 38. The eighth andninth symbols sensed by the detector circuit 36 are used by the energydetermination circuit 40 to measure the signal strength of the otherantenna 32 a, 32 b. In this manner, the antenna 32 a, 32 b with thestrongest signal is determined for further receipt of the data unit. Theselection of the proper antenna 32 a, 32 b decreases the data unit lossrate, decreases the total transmission delay, and increases the systemthroughput.

The tenth symbol would then need to be used to perform CFOD and timingsynchronization in order to determine the necessary calculations withthe short training sequence of ten symbols. Unfortunately, it is notpossible to perform either the CFOD nor the timing synchronization witha single symbol. In addition, if the other antenna 32 a, 32 b has theweaker energy then a symbol duration is required to switch back to thefirst antenna 32 a, 32 b from which to receive the remaining symbols.Unfortunately, no additional symbols are available during the shorttraining sequence from which to determine CFOD and timingsynchronization if the tenth symbol is used for switching. Also, mostsystems switch between the antennas 32 a, 32 b periodically so the firstsymbol of the short training sequence may arrive during the switchingtime. Accordingly, the first symbol detected may be the second or latersymbol. Similarly, this limits the available symbols within the shorttraining sequence from which to determine the necessary parameters.

After consideration of the typical implementations of antenna diversity,one would come to the realization that the designers of the P802.11Astandard did not consider the number of symbols necessary during theshort training sequence for antenna diversity using a single detectorcircuit. The apparent solution is to use multiple detection circuits,one for each antenna 32 a, 32 b, or otherwise do not implement antennadiversity.

One event that can result in a loss of a data unit is a false alarm. Afalse alarm occurs when the presence of a signal is “detected” eventhough there is no actual signal. A false alarm usually sets thedetecting circuit into a “busy” mode and starts a sequence of functions(decoding). As this sequence of functions is being performed by thedetecting circuit, the detecting circuit may determine that there hasbeen a false alarm and revert back to a “standby” mode. If the detectingcircuit has received no data units during the period in which thereceiving device is in the “busy” mode, then there will not be any lostdata units as a result of the false alarm. However, if a data unit hasbeen received during the “busy” mode, the detecting circuit might allowthe data unit to go undetected thereby contributing to the loss of adata unit. It may be observed that a false alarm potentially contributesto the loss of a data unit if the detecting circuit goes into a modewhere no further symbol detection is performed or when the detectingcircuit assumes that the long symbol will follow exactly after 10 shortsymbols. In contrast, the present inventor determined that the detectingcircuit should perform continuous monitoring of the channel which willdecrease the impact of false alarms. Also, the long symbol transitionshould be detected independently of the number of short symbols that aredetected to further avoid any cost associated with the “busy” mode thatoriginates due to the false alarm. In this manner, the detecting circuitdoes not need to actually count, or otherwise keep track of, the numberof short symbols of the short training sequence. If the long symboltransition goes undetected, then the situation simply becomes anothermissed data unit detection. With this modification of the detectingcircuit the effects of false alarms can be minimized, especially in areceiving device with a short training sequence having a limited numberof symbols.

Disregarding the effects of a false alarm, a data unit is consideredlost if the detection of the data unit is missed or if the data unitcontains unrecoverable errors. Thus, the probability of a lost data unitmay be represented as (one antenna):

 Pr[a data unit loss]=P _(M)+(1−P _(M))Pr[Frame in error|Frame isdetected]  (1)

where P_(M) is the probability of a missed detection. The probability,Pr[Frame in Error|Frame is detected], depends on the channel,convolutional coding, and the interleaver.

A missed detection is considered to occur when the presence of thesymbols of the short training sequence are, for example, (1) notdetected, (2) not detected early enough so that the necessary functionscan be performed with the short symbols received, (3) or when thetransition to the long symbols is not detected.

The functions that should be performed during the short symbol reception(either directly or on a buffered signal) are the symbol detection, theAGC, the switching, the diversity selection if there are multipleantennas (energy determination), the CFOD, and the timingsynchronization. The CFOD and the timing synchronization can only bereasonably performed after the establishment of the AGC. Thus, it isimportant that the AGC be determined prior to CFOD and timingsynchronization. At least two short symbol durations are required toperform CFOD and timing synchronization. Under the assumption that AGCcan be performed in one short symbol, while using one antenna, thedetecting circuit needs to be able to detect the signal within at leastseven short symbols (10 total symbols—[AGC(one symbol)+(CFOD+TS) (twosymbols)]. Assuming a nominal operating signal-to-noise (S/N) ratio of 8dB in an Additive White Guassian Noise (AWGN) channel, the probabilityof a missed detection during a short symbol is about 10⁻⁴, while in aflat-fading Rayleigh channel, it is about 10⁻². Since there are at least7 short symbols during which the signal needs to be detected, theprobability of a missed detection during short symbol is 10⁻²⁸ in theAWGN channel and 10⁻¹⁴ in a flat fading channel. Even when the gainadjustment is increased to three (presuming AGC takes three symbols toperform) short symbols, the worst-case scenario of the probability of amissed detection is 10⁻¹⁰. Thus, the first term in equation (1) abovecan be neglected and hence a missed detection is very unlikely to occurin the case of a single antenna and thus can be effectively disregarded.

For the case of multiple antennas, equation (1) may be modified to thefollowing (equation 2):

Pr[a data unit lost]=Pr[missed detection on both antennas]+Pr[antenna 1is used for detection]Pr[frame in error|antenna 1 is used fordetection]+Pr[antenna 2 is used for detection]Pr[frame in error|antenna2 is used for detection]

Note that in equation (2) the last two terms on the right hand sideconsider the situation in which the signal is detected by only oneantenna at a time. Equation (2) disregards the effects of false alarmsbecause false alarms are highly unlikely in view of the aforementioneddetecting circuit modification. The first term of equation (2) isanalogous to the first term in equation (1). As before, a data unit isdeemed missed when the presence of the symbols of the short trainingsequence are, for example, (1) not detected, (2) not detected earlyenough so that the necessary functions can be performed with thereception of the short symbols, (3) or when the transition to the longsymbols is not detected. In order to minimize the probability of a miss,the functions that need to be performed during the reception of theshort symbols should be done in a timely manner. When the receiver is ina standby mode, the receiver will alternate between the two antennaswith signal collection on each antenna normally lasting for the durationof one short symbol. This also means that the receiver should wait forone short symbol before it starts detecting the signal (and get ready toperform the associated functions). When a symbol is detected, thereceiver needs to as before, three to five symbols to establish thesignal (switch, detect, AGC, CFOD, and time synchronization). In eithercase, there are at least five symbols during which the presence of thesignal needs to be detected. Thus, the probability of a miss by twoantennas is the same as for one antenna (nearly zero) and may bedisregarded. Also, the first multiplicative term in the second additiveterm in equation (2) (as well as the first multiplicative term in thethird additive term) depends on the signal level on the antenna withrespect to the signal level on the other antenna and/or the time line.In essence, the antenna with the greatest energy level is selected.

With the establishment that missed detections and false alarms areunlikely, and in general can be disregarded, the present inventorestablished the requirements desirable for antenna selection diversitywith two (or more) antennas. A pair of symbols are typically used forthe detection of the presence of the signal (optional), namely, onesymbol (or more) for the first antenna and another symbol (or more) forthe second antenna. Also, a pair of symbols are used for AGC adjustment,namely one symbol (or more) for the first antenna and another symbol (ormore) for the second antenna. In addition, a pair of symbols (or more)for each antenna are used for energy determination. One symbol (or more)is normally used for the switching from one antenna to another. Thus atleast nine symbols are needed, namely, for detection (two), for AGC(two), for energy determination (four), and for switching (one). Thisonly leaves one additional symbol which the present inventors determinedis not sufficient to perform the CFOD or the timing synchronization, aspreviously described. The additional symbol is useful in the event ofswitching during the first symbol. Unfortunately, at least two symbols(or more) for each antenna are used for CFOD and timing synchronization,namely, two symbols for the first antenna and two symbols for the secondantenna.

After consideration of the aforementioned timing requirements togetherwith the present inventor's realization of the unlikeliness of falsealarms with continuous monitoring and the extremely low probability ofmissing a data unit, the present inventor came to the furtherrealization that the CFOD and the timing synchronization may beperformed once for each antenna together with the energy determinationto alleviate the restrictions imposed by the ten symbols of the shorttraining sequence. Referring to FIG. 5, the preferred technique includessymbol detection by a first antenna, AGC, then the combination of energydetermination, CFOD, and timing synchronization. Then the systemswitches to a second antenna during the subsequent symbol. Then symboldetection, AGC, then the combination of energy determination, CFOD, andtiming synchronization is performed with symbols from the secondantenna. The first symbol may be missed if a switch is performed when itis received. The number of short symbols used with the technique of FIG.5 is nine short symbols, which is less than the ten symbols available.In addition, if the first symbol is missed, then only nine short symbolsare used which is still permitted by the P802.11A standard. Ifnecessary, the antenna may be switched to the first antenna if it hasgreater energy during the last symbol or after the short trainingsequence. It is to be understood that the determination of theparameters may likewise be performed on buffered signals, if desired, sothe order of calculation may differ somewhat. In addition, the symboldetection is optional. Also, the detection and/or determination of theparameters may be done between the symbols, if desired.

Referring to FIG. 6, a flowchart for implementing the preferredtechnique includes starting at block 50. Block 52 switches to antenna A.Block 54 sets variables E=0, E_(old)=0, and F=0. E represents the energydetected by the currently selected antenna. E_(old) represents theenergy detected by the previously selected antenna. F is a flag. Block56 integrates any energy received on the selected antenna (antenna A)during a short symbol duration and stores the result in E. In otherwords, block 56 senses the presence of a symbol and some measure of itsenergy. Block 58 determines if the received energy in E is less than afirst threshold T1. If E is less than T1 then block 60 determines if theflag F is 1. If the flag F is not one, then E is set to 0 by block 62and block 64 switches to the other antenna. Setting E to 0 reinitializesthe energy detected back to zero. The loop of blocks 56, 58, 60, 62, and64 alternatively switch between the antennas until sufficient energy isdetected indicating a valid data unit at block 58 by E not being lessthan T1.

When sufficient energy is detected control is passed from block 58 toblock 70 which performs AGC, then collects the signal energy into E,determines the CFOD, and determines the timing synchronization. Block 72determines if E is less than a second threshold T2, which if true, thenthe system branches to block 60. If F is not equal to 1 then E is set tozero and the antenna is switched at block 64. This is representative ofreceiving noise or simply insufficient energy to process the rest of thedata unit. If block 72 determines that E is not less than a secondthreshold T2, then the system saves the parameters, AGC, E, CFOD, andtiming synchronization, at block 74. Block 76 checks to see if F isequal to 1, which if not true, then control is passed to block 78. Block78 sets F equal to 1 and E_(old) equal to E. Setting F equal to 1indicates that a first set of symbols from a first antenna has beenprocessed having a sufficient energy and that the other antenna shouldnow be checked. Setting E_(old) equal to E saves the energy determinedfrom the first antenna so that it may be later compared again the energyfor the other antenna.

Now that the system has detected a set of symbols with sufficient energyon the first antenna, the system will check the other antenna. Block 64switches to the other antenna and block 56 integrates and saves theresulting value in E. If E (for the other antenna) is less thanthreshold T1 then block 58 branches to block 60, which in turn branchesto block 80. This result indicates that no valid signal was detected onthe other antenna. Block 80 switches back to the first antenna which hada valid set of detected symbols. Control is then passed to block 82 forthe detection of the subsequent long symbol detection.

If E (for the other antenna) is not less than the threshold T1 thenblock 58 branches to block 70, which performs AGC, energy determination,CFOD, and timing synchronization. If E is less than threshold T2 theninsufficient energy was detected for the other antenna and control ispassed to block 60. Block 60 then passes control (F=1) to block 80 whichswitches to the first antenna with the valid data and passes control toblock 82 which does long symbol detection. This represents valid datafor the first antenna while the data is not sufficient for the otherantenna. Accordingly, the first antenna is used for the subsequent dataunit.

If E (for the other antenna) is not less than T2 then the parameters,AGC, E, CFOD, and timing synchronization, are saved at block 74 (notover writing the parameters from the first antenna). Block 76 passescontrol (F=1) to block 84 which determines which antenna has the greaterenergy. If the currently selected antenna (other antenna) has thegreatest energy then control is passed to block 82. If the currentlyselected antenna (other antenna) does not have the greatest energy thencontrol is passed to block 80 to switch to the first antenna andsubsequently to block 82. In this manner the antenna with the greatestenergy is selected, all of which is performed within 10 short symbols,or otherwise before valid data is received from the long trainingsequence.

It is to be understood that the aforementioned techniques may likewisebe applied to any other communication system.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

What is claimed is:
 1. A method of adjusting a receiving device inresponse to sensing symbols comprising: (a) adjusting an automatic gaincontrol in response receiving a first symbol of a data unit from a firstantenna; (b) after adjusting said automatic gain control receiving atleast a second and a third symbol of said data unit and in responsethereto (1) calculating a first energy of at least one of said secondand third symbol, (2) calculating a first frequency offset of at leastone of said second and third symbol, and (3) calculating a firsttemporal offset of at least one of said second and third symbol; (c)adjusting said automatic gain control in response receiving a fourthsymbol of said data unit from a second antenna; (d) after adjusting saidautomatic gain control receiving at least a fifth and a sixth symbol ofsaid data unit and in response thereto (1) calculating a second energyof at least one of said fifth and sixth symbol, (2) calculating a secondfrequency offset of at least one of said fifth and sixth symbol, and (3)calculating a second temporal offset of at least one of said fifth andsixth symbol; and (e) selecting at least one of said first and secondantenna based upon a comparison between said first and second energy. 2.The method of claim 1 wherein said automatic gain control is a singleautomatic gain control circuit.
 3. The method of claim 1 wherein saidfirst, second, third, fourth, and fifth symbols are in a non-sequentialorder.
 4. The method of claim 1 wherein said automatic gain controladjusts the amplitude variation of the received symbols at the input toan analog-to-digital converter.
 5. The method of claim 1 wherein saidfirst frequency offset and said second frequency offsets are coarseoffset frequency determinations.
 6. The method of claim 1 wherein saidfirst temporal offset and said second temporal offset are timingsynchronization.
 7. The method of claim 1 wherein said comparisonbetween said first and second energy is a magnitude determination. 8.The method of claim 1 further comprising sensing an initial symbol fromsaid first antenna prior to said first symbol.
 9. The method of claim 8further comprising sensing an intermediate symbol from said secondantenna prior to sensing said fourth symbol and after sensing said thirdsymbol.
 10. The method of claim 9 further comprising switching from saidfirst antenna to said second antenna prior to receiving said fourthsymbol and after receiving said first symbol.
 11. The method of claim 10wherein said switching occurs during one symbol duration.
 12. The methodof claim 10 wherein said first symbol, said second symbol, said thirdsymbol, said fourth symbol, said fifth symbol, said sixth symbol, saidswitching, said initial symbol, and said intermediate symbol occurswithin a time duration of nine symbols.
 13. The method of claim 12further comprising switching from said second antenna to said firstantenna after receiving said sixth symbol and no later than a tenthsymbol of said data unit.
 14. The method of claim 8 wherein said sixthsymbol is received no later than a tenth symbol of said data unit. 15.The method of claim 9 wherein said sixth symbol is received no laterthan a tenth symbol of said data unit.
 16. The method of claim 10wherein said sixth symbol is received no later than a tenth symbol ofsaid data unit.
 17. The method of claim 12 wherein said sixth symbol isreceived no later than a tenth symbol of said data unit.